The present invention relates to the field of multi-turn variable resistance devices, and more particularly, to a ratcheting mechanism for multi-turn variable resistance devices employing a worm screw.
Multi-turn variable resistors having a lead or worm screw adjusting mechanism are employed in many varied applications. Very often, such resistors are used in applications where they are adjusted infrequently after the circuit has been initially adjusted or "trimmed."
When readjustment of the resistors is required, the resistors are generally designed so that one turn of the lead or worm screw rotates the contact member only a short distance relative to the conductive and resistive tracks of the variable resistor. Accordingly, if a major readjustment of the resistance device is required, the lead screw must be rotated through many turns in order to obtain the desired new resistance value.
Typically, variable resistors of the type described, are generally of relatively small size whereby the size and strength of the individual components of the device are necessarily reduced and therefore are more susceptible to damage. Adjustment of the resistance via rotation of the worm screw is generally accomplished by the use of a screwdriver or similar tool.
Very often, the resistive and conductive tracks are arcuate and the contact bearing member is rotated relative thereto when the desired readjustment of the resistance device is required. Although the conductive and resistive tracks are generally arcuate, they typically do not comprise a complete circle, but rather comprise a major sector thereof, for example 320.degree..
For this reason, stop means are employed to limit rotation of the contact bearing member at either end of the resistive and conductive tracks.
In the absence of clutch mechanisms, the only indication that an operator receives when the contact bearing member has reached the end of its travel would be a sudden increase in the torque required to turn the worm screw. This might happen quite abruptly and if the operator were not expecting same, he might apply an excessive amount of torque to the worm screw whereby damage to the stops and/or gear teeth of the contact bearing member and worm screw might occur.
Accordingly, to prevent the foregoing damage, clutch mechanisms of various types are employed to disengage the lead screw when the contact member reaches the end of its travel. To prevent damage from occurring to the various elements of the variable resistor, it is essential that such clutch mechanisms be highly reliable in operation and of long lasting design.
Examples of prior art clutch mechanisms are disclosed in U.S. Pat. Nos. 3,115,614; 3,179,910; 3,242,452; and 3,768,325. The mechanisms employed in U.S. Pat. Nos. 3,115,614 and 3,242,452 utilize a ratcheting mechanism in which a portion of the teeth of a worm gear have been omitted to form a blank area. A member having two resilient, relatively short strips is located in the blank area to engage the threads of the worm screw when the worm gear teeth are rotated out of engagement with the threads of the screw. At this time, further rotation of the worm gear is prevented by means of stop means. The disadvantage of such a clutch mechanism involves the use of the relatively short resilient strip members which, after repeated usage, may become over-stressed and thereby break.
The clutch mechanisms disclosed in U.S. Pat. Nos. 3,179,910 and 3,768,325 employ a resilient stop whereby a worm gear bearing the contact member is caused to oscillate when the contact member reaches either end of the resistive and conductive tracks. If minute particles of dirt or other fine material are on either the resistive track or the contact member, electrical noise will be generated as the gear and contact member are oscillated.